The present invention relates to a cylindrical reactor suitable for exothermic heterogeneous reactions comprising injection of a gaseous reactant and a liquid reactant and especially for hydroformylation reactions. The invention further relates to a continuous process for producing aldehydes and/or alcohols having 6 to 20 carbon atoms by hydroformylation of olefins with synthesis gas in the presence of a transition metal catalyst homogeneously dissolved in the liquid reaction medium in such a reactor.